An optical system comprises an optical fiber with gain producing core with an index of refraction n1, surrounded by at least one cladding with an index of refraction n2, said cladding including at least one index reduced area with an index of refraction n2, such that n1>n2>n2, the core propagating a signal at a spatial fundamental mode at a signal wavelength λ1 and at a power level sufficient to generate optical power at a wavelength λ2, where λ2>λ1, and the at least one index reduced area in combination with the core provide has at least one cut-off fundamental spatial mode wavelength λC, and λ1<λC and λ2>λC.
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1. An optical system comprising:
(i) an optical fiber with gain producing core with an index of refraction n1, surrounded by at least one cladding with an index of refraction n2, said cladding including at least one index reduced area with an index of refraction n2, such that n1>n2>n2, said core propagating a signal at a spatial fundamental mode at a signal wavelength λ1 and at a power level sufficient to generate optical power at a wavelength λ2, where λ2>λ1, and said at least one index reduced area in combination with said core provides at least one cut-off fundamental spatial mode wavelength λC, and λ1<λC and λ2>λC, wherein (i) said at least one index reduced area does not surround said core, and (ii) said core and said at least one index reduced area are situated in close proximity to one another such that the distance between adjacent edges of said core and said at least one index reduced area is less than the core diameter.
18. A method of operating an optical system, said method comprising the steps of:
(a) providing optical fiber with (i) gain producing core having an index of refraction n1, (ii) at least one cladding surrounding the core and having an index of refraction n2, the cladding including at least one index reduced area adjacent to said core, said at least one index reduced area having an index of refraction n2, such that n1>n2>n2, wherein (i) said at least one index reduced area does not surround said core, and (ii) said core and said at least one index reduced area are situated in close proximity to one another such that the distance between adjacent edges of said core and said at least one index reduced area is less than the core diameter, the optical fiber having at least one cut-off fundamental spatial mode wavelength λC;
(b) optically pumping the core and generating optical gain at a signal wavelength λ1 in the core of the optical fiber, and propagating a signal at a spatial fundamental mode at the signal wavelength through said core λ1 at a power level sufficient to generate optical power at a wavelength λ2, where λ2>λ1, and λ2<λC;
(c) preventing propagation of signal at the fundamental spatial mode at the wavelength λ2, while propagating signal at the fundamental spatial mode at the wavelength λ1, thereby minimizing power loss due to power shift from the signal of wavelength λ1 to signal of wavelength λ2.
2. The optical system according to
3. The optical system according to
4. The optical system according to
(i). a silica based, rare earth doped core having a first index of refraction n1;
(ii). a silica based inner cladding surrounding the core and having a second index of refraction n2, such that n1>n2, said inner cladding having a plurality of air holes extending longitudinally through the length of said optical fiber;
(iii) a silica based outer cladding surrounding said inner cladding and having a third index of refraction n3, such that n2>n3;
wherein said optical fiber supports a single polarization mode within the operating wavelength range such that SRS is suppressed and λ2 is outside single polarization band of the optical fiber.
5. The optical fiber according to
6. The optical system according to
7. The optical system according to
(i). a silica based, rare earth doped core having a first index of refraction n1;
(ii). a silica based inner cladding surrounding the core and having a second index of refraction n2, such that n1>n2, said inner cladding having a plurality of air holes extending longitudinally through the length of said optical fiber;
(iii) a silica based outer cladding surrounding said inner cladding and having a third index of refraction n3, such that n2>n3;
wherein said optical fiber supports a single polarization mode within the operating wavelength range such that λ2 is outside single polarization band of the optical fiber.
8. The optical system according to
9. The optical fiber according to
10. The optical fiber according to
11. The optical system according to
12. The optical system according to
(i). a silica based, rare earth doped core having a first index of refraction n1;
(ii). a silica based inner cladding surrounding the core and having a second index of refraction n2, such that n1>n2, said inner cladding having a plurality of air holes extending longitudinally through the length of said optical fiber;
(iii) a silica based outer cladding surrounding said inner cladding and having a third index of refraction n3, such that n2>n3;
wherein said optical fiber supports a single polarization mode within the operating wavelength range such that λ2 is outside single polarization band of the optical fiber.
13. The optical fiber according to
14. The optical fiber according to
15. The optical fiber according to
19. The method of operating the optical system according to
(ii) said cladding includes at least one air hole situated in proximity to said core, so that n2=1.
20. The method of operating the optical system according to
(i) the optical power in fundamental spatial mode corresponding to both λ2 and λ1 is generated in said core, the wavelength λ1 corresponds to the signal that experiences “three level” gain and the wavelength λ2 corresponds to the signal that experiences “four level” gain; and
(ii) said cladding includes at least one air hole situated in proximity to said core, so that n2=1.
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1. Field of the Invention
The present invention relates generally to optical systems that utilize optical fiber for propagating high power optical signal, and more particularly to lasers, laser amplifiers and oscillators with a rare earth doped optical fiber operating in high power regime.
2. Technical Background
Optical fiber has become a favorite medium for telecommunications due to its high capacity and immunity to electrical noise. Single clad rare earth doped optical fiber has been widely used in the field of optical amplifiers and fiber lasers. This type of fiber has relatively low capability of handling high power multimode optical sources due to the difficulty of efficiently coupling multimode light from a high power optical (light) source (also referred to herein as optical pump or pump) into the rare-earth doped fiber core.
To solve this problem and to increase the output power of fiber lasers, those of skill in the art utilize optical fiber with a double clad structure (referred herein as double clad optical fiber). Double clad rare-earth doped optical fiber is a fiber that has a core, an inner cladding layer surrounding the core and an outer cladding layer surrounding the inner cladding layer. Optical fibers with Yb doped cores and two cladding layers surrounding the core are disclosed, for example, in U.S. Pat. Nos. 6,477,307; 6,483,973; 5,966,491 and 5,949,941.
Double clad optical fiber has been used in applications requiring utilization of optical sources providing between 10 to 100 Watts of optical power, because double clad optical fiber is more efficient in retaining/utilizing optical power provided by the optical pump than single clad optical fiber. This higher efficiency is due to fiber's utilization of clad-to-core coupling of optical pump power. More specifically, rare-earth doped double clad optical fibers accept light from the optical pump into the inner cladding and then transfer light to the rare-earth doped core through the core to inner cladding interface, along the length of the optical fiber. Thus, the optical fiber converts a significant part of the multi-mode light propagated through the inner cladding into a single-mode output at a longer wavelength, by coupling the pump light into the rare-earth doped core.
The inner cladding of the double clad optical fiber has a higher index of refraction than the outer cladding, thus the pump energy is confined inside the inner cladding and is re-directed into the core. The optical fiber is optically active due to the presence of rare-earth dopant in the core, which can be excited to higher electronic energy levels when the optical fiber is pumped by a strong optical pump. The core is typically doped with at least one rare-earth element, for example, neodymium or ytterbium, to provide lasing capability in a single-mode output signal. Typically, a neodymium- or ytterbium-doped double-clad fiber is pumped with one or several high-power broad-area diode lasers (at 800 nm or 915 nm) to produce a single transverse mode output (at the neodymium four-level transition of 1060 nm or the ytterbium four level transition of 1030 nm-1120 nm, respectively). Thus, conventional double-clad arrangements facilitate pumping of the optical fiber using a multi-mode inner cladding for accepting and transferring pump energy to the fiber core along the length of the fiber. Cladding pumping can be utilized in fiber amplifiers, or employed to build high-power single mode fiber pump lasers.
In fiber laser applications rare-earths such as Nd, Yb or Er have three-level transitions that require at least 50% of total ions to be in the upper level (i.e., at least 50% inversion) to exhibit gain. These three level transitions result in production of gain (or lasing) in these wavelength ranges: 880-920 nm in Nd doped optical fibers, the 965-1020 nm in Yb doped optical fibers, 1510-1540 nm in Er doped optical fibers. All of these are earth ions have competing 4-level transitions that require much lower level of inversion to exhibit gain, as low as 3% to 4%. The wavelengths corresponding to the 4-level transitions are 1050-1100 nm transitions in Yb and Nd doped optical fibers, and the 1560-1600 nm transitions in Er doped optical fibers. For a given length of the optical fiber and a given double-clad geometry, four level transitions reach transparency (i.e. the condition when gain reaches the loss in the fiber) or laser threshold at a much lower pump power level than three-level transitions. Once the optical fiber exhibits gain or starts lasing as a 4-level system, it does not operate as a 3-level system, and provides the gain or lasing wavelength in an undesirable wavelength range.
The problem of unwanted 4-level lasing or gain is typically solved by adding filters such as dielectric filters to suppress gain in four-level transitions, or alternatively, by choosing a double-clad inner cladding area that “promotes” a higher pump power density to increase the local inversion. However, dichroic filters are difficult to make spectrally sharp enough so that no additional loss is added at three-level wavelengths. That is, such filters introduce power loss at the 3-level wavelengths. Furthermore, these dichroic filters add further complexity and expense to the overall system. Provision of higher pump density is typically achieved by utilizing optical fiber with the relatively small clad to core ratios. While this approach increases pump power density to promote three-level inversion it makes it difficult and/or inefficient to couple pump power into the optical fiber and add complexity to the pump power coupling system.
Furthermore, when the optical fiber generates and propagates a high power optical signal and when the optical power exceeds Raman threshold level, the signal light is shifted to longer wavelength via Simulated Raman scattering, inducing power loss at the operating wavelength and preventing further power buildup.
The problem of Simulated Raman scattering is typically solved by increasing the fiber core diameter and lowering its numerical aperture. However, optical fibers with low numerical apertures and large core diameters are very susceptible to the bend-induced losses. In addition, because of non standard dimensions, these fibers are difficult to handle because they are not compatible with existing fibers or fiber handling technologies (fiber cleavers and fusion splices). Finally, when such fiber is fusion spliced to an optical fiber with a smaller core diameter, the signal power does not couple effectively into a smaller core of the second optical fiber, and a large amount of signal power is lost at the splice due to mismatch between the core sizes.
Single polarization optical fibers are useful for ultra-high speed transmission systems or for use as a coupler fiber for use with, and connection to, optical components (lasers, EDFAs, optical instruments, interferometric sensors, gyroscopes, etc.). The polarization characteristic (single polarization) propagates one, and only one, of two orthogonally polarized polarizations within a single polarization band while suppressing the other polarization by dramatically increasing its transmission loss.
Definitions:
The following definitions and terminology are commonly used in the art.
Refractive index profile—the refractive index profile is the relationship between the relative refractive index (Δ%) and the optical fiber radius (as measured from the centerline of the optical fiber).
Birefringence—birefringence is the difference between the effective refractive indices of the two polarization modes.
Radii—the radii of the segments of the fiber are generally defined in terms of points where the index of refraction of the material used takes on a different composition. For example, the central core has an inner radius of zero because the first point of the segment is on the centerline. For a segment having a first point away from the centerline, the radius of the waveguide centerline to the location of its first refractive index point is the inner radius of that segment. Likewise, the radius from the waveguide to centerline to the location of the last refractive index point of the segment is the outer radius of that segment. For example, an down-doped annular segment surrounding the central core would have an outer radii located at the interface between the annular segment and the cladding.
Relative refractive index percent Δ%—the term Δ% represents a relative measure of refractive index defined by the equation:
Δ%=100×(ni2−nc2)/2ni2
where ni is the maximum refractive index of the index profile segment denoted as i, and nc, the reference refractive index, is taken to be the refractive index of the cladding layer. Every point in the segment has an associated relative index measured relative to the cladding.
In accordance with one aspect of the present invention, an optical system 5 comprises an optical fiber with gain producing core with an index of refraction n1, surrounded by at least one cladding with an index of refraction n2, the cladding including at least one index reduced area with an index of refraction n2′, such that n1>n2>n2′. The core propagates signal at a spatial fundamental mode at a signal wavelength λ1 and at a power level sufficient to generate optical power at a wavelength λ2, where λ2>λ1. The optical fiber has at least one cut-off fundamental spatial mode wavelength λC, wherein λ1<λC and λ2>λC.
In accordance with some of the embodiments of the present invention, a rare earth doped optical fiber is provided which exhibits single polarization properties within a Single Polarization Band (SPB). The fibers parameters are selected such that the SPB coincides with an operating wavelength λ1 and the wavelength λ2 is outside the SPB.
However, in some of the embodiments SPB is zero and the rare-earth doped fiber is not a single polarization fiber (SPF). In these embodiments, as long as the undesirable wavelength λ2 is filtered by the optical fiber because fiber cut-off wavelengths λC is such that λ2>λC, the optical system is allowed to provide gain or lase at the desired wavelength λ1.
One advantage of the optical system of the present invention is its capability to produce large amount gain or handling large amounts of optical power, thus being capable for use in a high power laser or an optical amplifier, while eliminating power loss due to SRS or “a four level “signal. Another advantage of the optical fiber of the present invention is that because it eliminates the need to use larger core diameter fiber in high power applications, making splicing to other optical fibers easier and more efficient.
In some embodiments of the optical system of the present invention the optical fiber is a single polarization fiber that is employed to propagate the signal wavelength of interest within the SPB and to eliminate propagation of optical power in wavelengths associated with either SRS or, in a three level system, to eliminate signals associated with the four level gain.
Additional features and advantages of the invention will be set forth in the detail description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts.
In accordance with one aspect of the present invention, an optical fiber 10 has a gain producing core 12 with an index of refraction n1, surrounded by at least one cladding 14 with an index of refraction n2. The cladding 14 includes at least one index reduced area 15 with an index of refraction n2′, such that n1>n2>n2′. The cross section of such exemplary fiber is illustrated, for example, in
For example, we will consider a double clad, ytterbium-doped fiber laser with a core diameter of 10 microns. We can estimate the requisite critical pump power Pocr to reach the Raman threshold from equation (1):
where Aeff is the effective area of the transverse mode field, Leff is the effective length of the fiber, and gR is the Raman gain. If the fiber has the following parameters: Aeff=78.5 m2Leff=20 m, gR=10−13 m/W, Raman power threshold is 628W. At this pump power level Raman scattering will generate optical power at the wavelength λ2, which is Stoke- shifted due to Stimulated Raman Scattering (SRS) with respect to λ1. In order to achieve output power beyond 628 W at the signal wavelength λ1 with this fiber, the Raman scattering must be frustrated. If the operating wavelength of the laser is λ1=1064 nm, the Raman scattered light will occur at approximately 440 cm−1, or 1116 nm. If the fiber is designed such that the fundamental cutoff wavelength λC occurs between these two wavelengths, at 1100 nm, for example, the Raman wavelength λ2 will be filtered out. Thus, loss due to stimulated Raman scattering is either eliminated or minimized.
According to at least one embodiment of the present invention the wavelength λ2 is Stoke—shifted due to Stimulated Raman Scattering (SRS) with respect to λ1; and the optical fiber utilizes at least one hole (ether air filled, or filled with a silica-based material of relatively low index of refraction with respect to that of the rest of the cladding) to propagate the signal at this wavelength (λ1) while filtering out the unwanted signal associated with the wavelength λ2. Alternatively, the wavelength λ1 corresponds to the signal that experiences “three level” gain and the wavelength λ2 corresponds to the signal that experiences “four level” gain; and the optical fiber utilizes at least one hole (ether air filled, or filled with a silica-based material of relatively low index of refraction with respect to that of the rest of the cladding) to allow propagate the signal at wavelength λ1 while filtering out signal at the wavelength λ2. This is illustrated in
In some embodiments, the optical fiber 10 is a single polarization fiber and has two fundamental spatial mode cut off wavelengths (i.e., λC1 and λC2) defining Single Polarization Band (SPB). The optical fiber is employed to propagate the signal wavelength of interest within the SPB and to eliminate propagation of optical power in wavelengths associated with either SRS or, in a three level system, to eliminate signals associated with the four level gain. More specifically, the fiber's parameters are selected such that the SPB coincides with an operating wavelength λ1 and the wavelength λ2 is outside the SPB, because that λ2 is larger than the second cut-off wavelength (i.e., λ2>λC, where λC=λC2).
In accordance to some other embodiments of the present invention, the SPB is equal to zero and the rare-earth doped optical fiber 10 is not a single polarization fiber. This is achieved, for example, by making holes 24, 26 small (less than 3 micron in diameter), or having relatively small core delta (less 0.2% and preferably less than 0.1%), or making the separation between core 12 and each hole greater than about 3 microns.
Such an optical fiber will have only one fundamental spatial mode cut-off wavelength λC, defined as the wavelength at which loss is 10 dB higher than the background propagation loss of the fiber. However, as long as λ1<λC and λ2>λC, the longer undesirable wavelength(s) λ2 is filtered out and the desirable wavelength λ1 is allowed to propagate through the optical fiber. Thus, this type of the optical fiber may also be employed to propagate the signal wavelength λ1 and to filter out propagation of optical power in wavelength λ2 associated with either SRS or, in a three level system, to filter out optical signal at the wavelength λ2 that associated with the four level gain.
Thus, one advantage of the optical system of the present invention is its capability to produce large amount gain or handling large amounts of optical power, therefore being capable for use in a high power laser or an optical amplifier, while eliminating power loss due to SRS or a “four level” signal. Another advantage of the optical system according to the present invention is that because it eliminates the need to use of larger core diameter fiber in high power applications, making splicing to other optical fibers easier and more efficient. Yet another advantage of the optical system according to the present invention is that it eliminates or minimizes the need to use of dichroic filters to eliminate 4-level signals.
According to one aspect of the present invention a method of operating an optical system comprises the steps of:
One embodiment of double clad single polarization optical fiber 10 of the optical fiber 10 in accordance with the present invention is shown schematically in
The outer protective coating 18 may be, for example, an organic coating which typically includes a softer primary coating and a harder secondary coating applied over the primary coating.
In this optical fiber shown in
It is preferable that the outer cladding 16 be relatively thin, with wall thickness less than 80 μm and preferably between about 5 μm and 35 μm. It is most preferable that the wall thickness of the outer cladding 16 be between about 10 μm to 25 μm. It is preferable that the diameter Dc of the fiber core 12 be about 5 μm to 20 μm, the inner cladding diameter DIN be about 125 μm to 2000 μm and more preferably about 125 μm to 1500 μm. It is even more preferable that DIN be about 125 μm to 350 μm. It is preferable that the diameter of the outer cladding diameter (DOUT) be about 145 to 2100 μm, more preferably between about 145 μm to 1600 μm and even more preferable that DOUT be about 145 μm to 500 μm. If the inner cladding 14 does not have a circular cross section, DIN is defined as the smallest distance from one side of the inner cladding's cross section to the oppositely situated side of the cross section. It is also noted that the outer cladding 16 may not be circular. If the outer cladding 16 is not circular, DOUT is defined as the smallest distance from one side of the outer cladding's cross section to the oppositely situated side of the outer cladding's cross section. It is preferable that the inner cladding's 14 cross-sectional area be at least 200 times larger than the cross sectional area of the core 12. It is even more preferable that the cross sectional area of the inner cladding 14 be between 300 and 3000 times larger than the cross sectional area of the core 12. For example, the cross sectional area of the inner cladding 16 may be 500, 700, 1000, 1200, 1500, 1600, 2000 or 2500 times larger than the cross sectional area of the core 12.
According to this embodiment, the fiber core 12 includes, in weight percent:
Rare earth
0.1 to 2.5 wt %;
P
0 to 5 wt %;
Al
0.5 to 15 wt %;
Ge
0.1 to 15 wt %;
F
0 to 1 wt %.
The rare earth dopants in the fiber core 12 provide active ions to enable either a gain or a lasing action. Exemplary rare earth dopants are Yb, Er, Nd, Tm, Sm and Tb. It is preferable that the amount of rare earth dopant in the core 12 be 0.5 wt % to 1.5 wt %. Phosphorus may be added to the core materials in order to lower the softening temperature of the core glass, which may be advantageous if the core is produced by the inside vapor deposition process. Phosphorus may also be utilized as a refractive index raising agent. However too much phosphorus (10% or more) provides nonlinearity through Stimulated Raman Scattering which may inhibit the lasing action. Aluminum may be added to the core as a de-clustering agent (for example, to de-cluster Yb, preferably at the ratio of Al to Yb of 5:1 to 10:1). The core 12 may also include Germanium which is an index raising dopant, and/or fluorine which is an index lowering dopant as well as a de-clustering agent.
The preferred ranges of the core 12 composition in weight percent are:
Rare earth
0.3 to 1 wt %;
P
0 to 2 wt %;
Al
2 to 8 wt %;
Ge
3 to 15 wt %; and
F
0.1 to 0.5 wt %.
The Yb-doped core 12 will laze at 1.03-1.11 micron range.
It is preferable that the inner cladding 14 contain 5 wt % to 30 wt % Ge in order to provide high NA. It is even more preferable that the inner cladding comprise 5 wt % to 20 wt % Ge. It is noted that 5 wt % to 10 wt % Ge works well for many applications.
It is preferable that the index lowering dopant of the outer cladding 16 comprises Fluorine and/or Boron in weight percent:
F
0.5 to 5 wt %;
B
0.5 to 20 wt %.
The amount of dopant(s) for the outer cladding 16 is chosen to preferably result in inner cladding NA of between 0.15 to 0.5. However, it is preferable that the outer cladding 16 contain at least one of of B or/and F. It is preferable that the amount of B is at least 3 wt %. It is preferable to have more than 1 wt % and more preferably more than 2 wt % of F along with more than 8 wt % of B in the outer cladding 16. It is preferable that the outer cladding 16 has less than 5 wt % of F, and less than 15 wt % of B. It is even more preferable that the amount of B and F be: 2 to 4 wt % of F and 3 to 15 wt % of B.
Other embodiments of the double clad optical fiber of the present invention are shown schematically in
The optical fiber core 12 is preferably elliptical, as shown in FIGS. 1D and 2A-2C, but may have either a circular, or other elongated shapes. Adjacent to the core and situated at least partially within the inner cladding 14 are at least two air holes 24, 26. The core 12, in conjunctions with the air holes 24, 26 renders this optical fiber a single polarization (SP) fiber. It is preferred that the aspect ratio (ratio of major to minor axis) of the elliptical core 12 be at least 1.5:1 and more preferably be between 2:1 and 10:1, because these aspect ratios improve birefringence of the core 12. However, as stated above, circular core geometry may also be utilized.
The core delta is less than 1% Δ and preferably less than 0.5% Δ. The numerical aperture NA of the core 12 is between 0.03 (for high power laser application) and 0.25 (for lower power application). The numerical aperture NA of the core 12 is defined as (n12−n22)1/2, where n1 is the index of refraction of the core 12 and n2 is the index of refraction of the inner cladding 14.
The silica based inner cladding 14 may have a circular outer perimeter, as shown in
In general, a double-clad structure that could be used in a fiber laser or in an amplifier includes two claddings. A first (inner) multi-mode cladding acts as a multi-mode pumping core. The inner cladding 14 is adjacent to the core 12 and a second (outer) cladding 16 surrounds the first or the inner cladding 14. The core 12 may be either single mode or multi-mode at the core lasing wavelength. The inner cladding 14 serves as a waveguide with a high numerical aperture NA for the input (pumping) light. That is, the inner cladding serves as a pump cavity. The larger the inner cladding diameter, the more pump light is coupled into the inner cladding from the optical source. The cross-section of the first multi-mode inner cladding (DIN is the shorter dimension of the inner cladding as seen in
The light from this light source is then coupled to a double clad optical fiber via high NA and large aperture lenses. With this approach one can obtain 85-90% of coupling efficiency.
The invention will be further clarified by the following examples.
Core 12: 0.6 wt % Yb2O3; 4.5 wt % AL2O3; 3.0 wt % GeO2; 0.2 wt % F;
Inner cladding 14: 8.5 wt % GeO2;
Outer cladding 16: 9 wt % B and 2.7 wt % F.
The amount of each dopant is optimized to ensure the high laser efficiency. The preferred inner cladding shape is not circularly symmetric, thus maximizing the pump absorption.
It will be apparent to those skilled in the art that variations and modifications can be made to the present invention without departing from the scope of the invention. For example, although step index structures are show, other graded index structures may be employed. Moreover a ring structure may be added to the fiber profile as well and would still function acceptably. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.
Li, Ming-Jun, Wang, Ji, Walton, Donnell Thaddeus, Zenteno, Luis Alberto
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